Method of extracting copper values from copper bearing mineral sulphides



METHOD @F EXTRACTING COPPER VALUES giiggiisCflPPER BEARING MINERAL SUL-Frank Arthur Forward, Vancouver, British Columbia, Canada, assignor toSherritt Gordon Mines Limited, goronto, (Butario, Canada, a corporationof Ontario,

anada No Drawing. Application September 1, 1954 Serial No. 453,680

3 Claims. (Cl. 75-117) This invention relates to a method of extractingcopper values from material containing copper. The method isparticularly directed to the extraction and recovery of copper fromcopper bearing mineral sulphides.

This application is a continuation-in-part of my application Serial No.247,751, filed September 21, 1951.

Pyrometa-llurgical methods of extracting copper values from copperbearing mineral sulphides are well known and are widely used. Ingeneral, such methods involve the steps of smelting the sulphides in anoxidizing atmosphere to reduce the sulphur content and form a matte. Thematte is charged into a converter and the residual sulphur is removed toform a crude metal. The crude metal is refined, such as by electrolysis,to form substantially pure copper suitable for casting into commercialshapes. This conventional pyrometallurgical method has the disadvantagesof high capital and operating costs and losses of valuable metals infumes and slags.

Hydrometallurgical methods of extracting copper values from copperbearing ores and concentrates are also known and have been employed incommercial scale operations. Known hydrometallurgical methods for extracting copper values from ores and concentrates have the disadvantagesthat they are restricted to the treatment of oxidized ores or, in thetreatment of sulphide ores, such as leaching in place, they are slow andresult in wasteful losses of metal values and reagents. Other knownhydrometallurgical methods require a controlled oxidizing and/orreducing roasting operation prior to the leaching step.

It has been found that copper values can be extracted and recovered fromcopper bearing mineral sulphide ores and concentrates with relativelylow capital and operating costs and with a high degree of extractiveefliciency by leaching the copper bearing mineral sulphides with astrong aqueous ammonia solution at a temperature and pressure aboveatmospheric temperature and pressure in the presence of an oxygenbearing, oxidizing gas.

The method of treating copper bearing mineral sulphides of the presentinvention comprises, in general, the steps of dispersing finely dividedcopper bearing mineral sulphides in a strong aqueous ammonia solution toform a slurry which contains free ammonia in amount at least sufiicientto combine with the copper values to form amrnines, actively agitatingthe slurry in a reaction vessel maintained at a temperature above 80 F.and at a pressure above atmospheric pressure, feeding an oxygen bearing,oxidizing gas into the agitated mixture at a rate sufiicient to produceand maintain a partial pressure of oxygen above 5 pounds per square inchwhereby copper values in the mineral sulphides are converted to and aredissolved in the aqueous ammonia solution as soluble copper compoundsand at least one oxidizable sulphur compound of the group polythionatesand thiosulphate is formed in the solution, separating undissolvedresidue from the copper bearing leach solution, reacting the clarifiedsolution with a free oxygen United States Patent 0 2,822,263 Fatente dFeb. 4, 1958 bearing gas for a-period of time sufficient to convertoxidizable sulphur compounds to a sulphur compound of the group ammoniumsulphate and ammonium sulphamate, reducing the free ammonia content ofthe solution, and thereafter reacting the ammoniacal leach solution atelevated temperature and at superatmospheric pressure and under apartial pressure of a sulphur free reducing gas for a period of timesuflicient to convert dissolved copper values to and precipitate themfrom the solution as copper metal powder, and separating precipitatedcopper metal powder from the solution.

The leaching stage involves the step of dispersing finely pulverizedcopper bearing mineral sulphides in a strong aqueous ammonia solution.Factors which affect the rate and the efficiency of the extraction ofthe copper values contained in the mineral sulphides and theirdissolution in the leach solution are the size of the mineral sulphideparticles, pulp density, or the percentage of solid particles in themixture, temperature, pressure, agitation, time of retention, and theammonia and ammonium sulphate concentration.

Copper bearing mineral sulphides to be treated by the method of thisinvention preferably are finely pulverized. When mixed -with the strongaqueous ammonia leach solution to form a slurry, a relatively uniformdispersion of particles in the solution is obtained in which maximumsurface area of the particles is exposed to the oxygenbearing oxidizinggas and the strong aqueous ammoniacal solution. it has been found thatwhile the size of the particles may vary widely, the reaction rate andthe extraction of copper values are best when the mineral sulphides areof the order of from about 35 to 325 mesh Standard Tyler screen. Mineralsulphides received from a preliminary concentrating process, such as aflotation process, may be of the order of from about 100 to 325 mesh andcan be passed directly into the leaching stage. Mineral sulphides whichotherwise have not been reduced in size, can be pulverized, such as bygrinding, to about 35 mesh or finer.

The pulp density, or the ratio of solids to solution in the mixture,influences the rate and the efiiciency of the extraction. The velocityof extraction is reduced as the pulp density is increased, the maximumpulp density being that at which the mineral sulphide particles can bemaintained as a relatively uniform dispersion in the solution. Theminimum pulp density is governed by operating economics to obtainmaximum extraction of copper values with a reasonable volume ofsolution. Thus, the pulp density can vary within reasonably broadlimits, a very satisfactory range being from about 15% to about 50%solids, depending on the copper content of the solids. This range ofpulp densities provides for a rapid rate of extraction of copper valuesand their dissolution in the leach solution.

The extraction of copper values from copper bearing mineral sulphides byleaching the mineral sulphides with strong aqueous ammonia in thepresence of an oxygen bearing, oxidizing gas, such as air, oxygenenriched air or oxygen will proceed over a relatively wide temperaturerange. The reaction proceeds with progressively increasing rapidity asthe temperature is increased above atmospheric temperature, that isabove about F. The upper temperature limit is governed by operatingeconomics having regard to the total pressure generated, that is, thepressure generated autogenously at the temperature at which the reactionis conducted plus the pressure of the oxygen bearing, oxidizing gas. Avery satisfactory temperature range is from about F. to about 300 F. Attemperatures below about 100 F., the time period necessary to obtainmaximum extraction of copper values is too long for economic operationand at temperatures above about 300 F. the total pressure is such as torequire relatively costly, high pressure reaction vessels.

. The leaching stage usually provides: the heat necessary for thereaction. If this heat is not sufficient, supplemental heat can beprovided by any suitable means. If the autogenous heat of the reactiontends to raise the temperature beyond desired limits, the temperaturecan be controlled by conventional means, such as by cooling coils.

The total pressure at which the reaction is conducted is the pressureautogenously generated by the temperature plus the partial pressure ofthe oxygen bearing, oxidizing gas. The oxygen bearing, oxidizing gas canbe air, oxygen enriched air or oxygen with or without an inert gas. Thegas is, of course, fed into the reaction vessel under pressure. It ispreferred to employ a partial pressure of oxygen within the range offrom about 5 to about 100 pounds per square inch. A very satisfactoryoxygen partial pressure can be obtained by feeding air at about 100pounds per square inch into the reaction vessel.

If the oxygen bearing, oxidizing gas contains an inert gas, such asnitrogen, as in air, a constant withdrawal of gas from the reactionvessel should be maintained. Withdrawn gas can be passed through apressure absorption tower wherein ammonia is recovered by washing thegas with water. a

The time of retention is determined to obtain maximum extraction ofcopper values and their dissolution in the leach solution, having regardto the fineness of the particles, the ratio of liquid to solids, thetemperature and pressure at which the reaction is conducted, and theagitation. Normally, it is preferred to operate the method underconditions of temperature, pressure and agitation to obtain maximumextraction of copper values, that is from about 96% to about 99% of thecopper contained in the mineral sulphides with a time of retention offrom 4 to 11 or 12 hours.

The mixture of mineral sulphides and aqueous ammonia solution isagitated sufficiently to maintain a relatively uniform dispersion ofparticles in the solution to produce maximum dispersion of gas in theliquid and to obtain a high concentration of oxygen at the liquid-solidinterface. It is believed that the oxygen present in the gas phasediffuses through the gas-liquid interface into the liquid and in theliquid phase to the surfaces of the mineral sulphide particles. Also, itis possible that copper ions diffuse in the particles to the surfaceswhere they are converted to copper amines which are soluble in the leachsolution. At the same time, oxygen adsorbed on the surfaces of theparticles may partially diffuse into the particles to facilitate theextraction of copper values. The extraction of copper values from themineral sulphides is thus facilitated by the agitation of the mixture.

The operation of the method is very simple. Finely pulverized copperbearing mineral sulphides were dispersed in a strong aqueous ammoniasolution, about one part 28% ammonia to two parts water. It is preferredto employ a strong or concentrated aqueous ammonia solution and theremust be at least sufiicient free ammonia present in the solution tocombine with copper values as they are oxidized to form copper amineswhich are soluble in the leach solution. As excess ammonia is recoveredin the solution treatment stages, a substantial excess of ammonia isusually employed in the leaching stage. For example, very satisfactoryresults are obtained in the use of an ammoniacal solution whichcontains, during the leaching operation, from 60 to 100 grams of freeammonia per litre. A pulp mixture of mineral sulphides and aqueousammonia, about 25% solids, by weight, was heated in a reaction vesselfor a period of about five hours. The mixture was actively agitated. Airwas fed into the reaction mixture and anhydrous ammonia was supplied tomaintain a free ammonia concentration of about 90 grams per litre. Atthe end of the reaction period, it was found that about 95% of thecopper had beenextracted from the mineral sulphides and dissolved in theleach solution.

The following examples illustrate the operation of the method of thepresent invention. In each instance a copper sulphide concentrate wasemployed which had the following average analysis:

This concentrate was ground to a particle size of about 99.5% minus 325mesh Standard Tyler screen. Finely pulverized concentrates were chargedinto a pressure vessel with strong ammonia leach solution whichcontained about 100 grams of ammonia per litre of solution, to form amixture containing about 25% solids. The leaching step was conducted ata temperature of about 175 F. and under a pressure of about pounds persquare inch, air being employed as the oxygen bearing, oxidizing gas andanhydrous ammonia was continuously supplied to maintain a free ammoniaconcentration of approximately 90 grams per litre. The leaching stagewas continued for about 9 hours after which the mixture was dischargedfrom the reaction vessel and the undissolved residue was separated fromthe leach solution by filtration.

The following results were obtained:

Leach solution: Grams per litre Copper 39.87 Zinc 6.80 Sulphur (total)50.9 Sulphur as S0, 35.7 Sulphur as S 0 0.21 Sulphur as S 0 0.13 Ammonia(total) 114.1 Ammonia (free) 94.5 NH 7.14 Gold Trace Silver Trace Theterm NH free is defined as NH determined by direct titration withsulphuric acid, and the term NH total" includes NH free plus the ammoniacontained in ammonium compounds.

The residue, after washing, contained about 1.6% copper, about 1.2%zinc, about 52.7% iron, about 0.34 ounce per ton' gold and about 11.9ounces per ton silver. About 97% ofthe copper and about 90% of the zincwere extracted from the concentrate and dissolved in the leach solution.The iron values contained in the concentrate were converted to a ferricoxide or ferric hydrate which reported in the undissolved residue. Also,substantially all the gold and silver values reported in the residuewhich was in ideal condition for the recovery of such precious metalvalues, such as by cyaniding.

The presence of ammonium sulphate in the leach solutron expedites theextraction and substantially increases the efficiency of the extraction.

A copper-zinc sulphide concentrate which contained about 24% copper,about 4.5% zinc, about 36% sulphur and about 31% iron Was ground to afineness of about 100% minus 200 mesh. About one part by weight ammoniumsulphate to two parts by weight concentrate was mixed with theconcentrate. The mixture of concentrate and ammonium sulphate wasdispersed in strong aqueous ammonia solution, about 100 grams per litreammonia, to form a mixture which contained about 25% solids by weight.The leaching stage was conducted at a temperature ofabout 225 F. andatan oxygen partial pressure of about 20 pounds per square inch above thepressure autogenously. generated at the temperature of the reaction fora period of about three hours. At the end of the three hour leachingperiod, it was found that about 97.3% of the copper, about 94% of thezinc and about 72% of the sulphur had been extracted from the mineralsulphides and had dissolved in the leach solution. About 95% of thecopper was recovered with a leaching period of five hours without addedammonium sulphate.

It is possible to conduct the leaching stage in a plurality of stepswith the leach solution flowing in countercurrent to the fiow of mineralsulphides. Fresh mineral sulphides and leach solution from a precedingstep are charged into and pregnant leach solution containing dissolvedmetal values is discharged from the first step and partially leachedmineral sulphides and fresh leach solution are charged into andundissolved residue is discharged from the final step. Partially leachedmineral sulphides are passed in sequence from the first step to thefinal step from which, after washing, they are finally discharged towaste or passed to subsequent treatment for the recovery of undissolvedmetal values. Leach solution is passed in countercurrent to the flow ofmineral sulphides from the final step to the first stepfrom which it isdischarged in ideal condition for treatment for the recovery ofdissolved .metal values.

The following example illustrates the operation of the method employinga two-stage counter-current flow of solution and solids as describedabove to produce a pregnant leach solution containing polythionate ionshaving more than two sulphur atoms in their molecular structure, such astrithionate and tetrathionate, and/or thiosulphate. The ore concentratecontained about 6.5% nickel, about 5.5% copper, about 30% iron, about30% sulphur and about 25% insoluble matter. The concentrate was groundto about 80% minus 200 mesh Standard Tyler screen. The pulverizedconcentrate was charged into the first stage wherein it was retained forabout four hours at a temperature of about 175 F. and at a pressure ofabout 100 pounds per square inch. Undissolved residue was separated fromthe pregnant leach solution and passed to the second stage wherein itwas retained for about 12 hours at a temperature of about 180 F. and apressure of about 110 pounds per square inch. Undissolved residue fromthe second stage Was separated from the solution and discharged from theprocess. Leach solution from the second stage was passed to the firststage. A free ammonia concentration 'of about 90 grams per litre wasmaintained throughout both leaching stages. Air was fed into the secondstage and passed to the first stage. Ammonia was separated from the gasdischarged from the first stage and re-used in the leaching operation.

Pregnant leach solution discharged from the first stage contained:

Grams per litre Nickel 22 Copper 18 Sulphur (total) 85.5 Sulphur assulphate 52.0 Sulphur as thiosulphate 6.5 Sulphur as thionate 10.3Ammonia (total) 145 Ammonia (free) 90.0 NH 8.1

About 90% of the nickel, about 91% of the copper and about 75% of thesulphur were extracted from the stage is conducted. Normally, oxidizablesulphur com-- pounds, such as polythionates and thiosulphate, are formedin the early part of the leaching stage. As the leaching period isextended these oxidizable sulphur com- 5 pounds tend to convert toammonium sulphate and/or ammonium sulphamate. It is essential that thesolution passed to the reduction stage, described in detail hereinafter,be substantially free from oxidizable sulphur compounds, otherwise thecopper metal powder precipitated 10 in the reduction stage iscontaminated with sulphur to the extent that it does not meet marketspecifications.

The deleterious efiect of and the problem presented by the formation andthe presence of oxidizable sulphur compounds in the copper bearing leachsolution can be overcome by extending the leaching stage for a period oftime sufiicient to oxidize the oxidizable sulphur compounds to ammoniumsulphate and/or ammonium sulphamate, or, alternatively, by reacting thepregnant leach solution, after separation of undissolved residue, withan oxygen bearing, oxidizing gas, such as air, or oxygen enriched air,for a period of time suflicient to oxidize the oxidizable 'sulphurcompounds substantially completely.

Thus, the leaching stage can be conducted as a single stage operationwhich is continued to obtain optimum extraction of copper values fromthe mineral sulphides and optimum oxidation of oxidizable sulphurcompounds. Alternatively, the leaching stage can be conducted in aplurality of steps with the leach solution flowing in counter-current tothe flow of mineral sulphides. This latter modification has theimportant advantage that the copper content of the solution withdrawnfrom the first or adjustment leaching stage can be closely controlledand optimum extraction of copper values from the mineral sulphides canbe obtained in the final step or steps. Oxidizable sulphur compoundspresent in the solution can be oxidized to ammonium sulphate and/ orammonium sulphamate after the solution is withdrawn from the leachingstage. 1

Also, a co-current leaching stage can be employed instead of acountercurrent leach. This technique involves passing the leach solutionand mineral sulphides co-currently from one reaction vessel to anotherusing a plurality of steps. The leach is continued until the desiredextraction of copper values is obtained and thiosulphate and/orsulphamate compounds.

It is found, in the operation of the leaching stage hereinbeforedescribed that values of metals such as copper, 5 nickel, cobalt, zincand cadmium are extracted from the mineral sulphides and are dissolvedin the leach solution. Iron values report in the undissolved residue asferric oxide or ferric hydrate. Values of metals which are relativelyinsoluble in the leach solution, such as lead, also report in theresidue. Undissolved residue can be separated from undissolved residuesuch as by filtration.

The pregnant leach solution, after oxidation of thiosulphate andoxidizable polythionate compounds and after separation of undissolvedresidue, is heated at or near its 60, boiling temperature, to reduce thefree ammonia content. Released ammonia can be recovered for re-use inthe leaching stage. As the ammonia content is reduced towards a molratio of ammonia to copper of about 3.6 to 1, there may be a minorprecipitation of basic copper salts. It is desirable to maintain in thesolution an ammonium sulphate concentration oftrom about 200 to about400 grams per litre. The presence of ammonium sulphate in the solutionfacilitates and improves the extraction of copper values from themineral sulphides and also minmizes the precipitation of basic coppersalts in the boiling stage. The ammonia content of the solutionpreferably is reduced until the mol ratio of ammonia to coper reachesabout 2.5 :1. The solution from this stage is acidified with sulphuricacid until the ammonia to copper ratio is about221. The solution is thenpassed to the and oxidizable polythionates are oxidized to sulphate.

amass reduction stage wherein the copper values are reduced to (3)Effect of NH concentration: and precipitated from the solution as coppermetal pow-, C auditions. der substantially free from impurities. Coppermetal. Temperature..- 350 F. powder can be separated from the solutionby filtration. g fggg7flffi I: gg nii'n tit s Copper values can beprecipitated from an aqueous 5 on concentration--- 45 g. solution byknown methods, such as by cementation or virfable' (NH) so Nu byelectrolysis. It has been found, however, that an am- 4 a 4 moniacalsolution prepared according to the method of NH [Cu Percent on thepresent invention is ideally adapted for reduction and v a precipitatedprecipitation of dissolved copper values by treating the solution with asulphur free reducing gas such as, for example, carbon monoxide or,preferably, hydrogen, at a temperature and pressure above atmospherictemperature and pressure.

It is found, when hydrogen is employed as a reducing gas, that verysatisfactory results are obtained in the reduction stage by employing anammoniacal solution in which the ratio of ammonia to copper is about 2to 1, that is a diammine solution, containing copper as Cu(N1I foundihgtp p g 1s i s wlth' Copper values are rapidly reduced to and precipitatedm Prescn e i 5 an from the solution as a fine powder substantially freefrom 112 3 35353 rano o a out to mo S ammoma impurities at a temperatureabove about 300 F. and at a partial pressure of hydrogen ofabout 200pounds (4) Effect of (NH4)2SO" per, square inch above the pressureautogenously gen- Conditions: 3

T e ature 50 F. erated at the temperature at which the reducing reactionsgg l 300 8.1. H2. is conducted. geaction time; 2g mi n utes.

In the. operation of the reduction stage, ammonlacal E gggi j vargiamasolutioncontaining from about to about 60 grams of copper per litre isfed into a reaction vessel adapted to withstand relatively highpressures, such as an autoclave, wherein it is heated to a temperatureabove about 300 F. and preferably between about 300 F. and 500 F. Thesolution in the reaction vessel is actively agitated and hydrogen is fedinto the reaction vessel in amount suf- 5 ficient to maintain a partialpressure of hydrogen of above I i f d h h eflect f monium sulphate atabout POunds P Square inch and Preferably from ammonia to copper ratioslower than 1.7:1 improves the 'abQut 100 flbflllt 500 Pounds P Squareinch. Under reduction of copper from ammoniacal solutions wheretheseconditions, copper values dissolved in the solution as h fi t i negativeat hi h ammonia t copper rapidly reduce to and precipitate from thesolution as metal 40 i powder substantially free from impurities.Factors which I may not b econemic t o ti th d tion stage fi cq thePrecipitation 0f pp metal Powder fYOm below about two grams of copperper litre. Small amounts 111011121031 Solutions y gaseous reduction aretemperature, of residual copper values can be converted to and precipi-Partifll pressule of hydrogen, ammonia concentration and tated from thesolution as copper sulphides, such as by ammomllm sulphate ConcentrationThe following treating the solution with hydrogen sulphide. Residualamples illustrate an influence of these factors: copper valuesprecipitated from the solution as copper 1. Effect of tfimpelature011316 fate of pp feducphides following the reduction stage can bereturned to 9 the leaching stage. Conditions: Copper metal powderrecovered from the reduction filfglggnrfifilgfil-i5 gt /l. 9'1 stage canbe marketed as such or it can be compressed Reaction i ffi emperormelted and cast into commercial shapes such as wire H2 partialpressure400 p. s. i. g. bars, cakes and ingots, and marketed as such;

The solution, after separation of dissolved metal values, percent cconsists essentially of ammonium sulphate. Crystalline Temperature p p eammonium sulphate can be recovered from this soluas metallic producttion by known means, such as by crystallization. A portion of thecrystalline ammonium sulphate from this stage 61.3 can be returned tothe leaching stage to facilitate and expedite the extraction of coppervalues from the mineral sulphides. Residual ammonium sulphate can bemarketed as such or can be passed to a regenerator for therecovery ofammonia.

The method of the present invention possesses a num- 2) Effect of Hpartial pressure:

Conditions:

Cu concentration45 g./l.

%eilnpetriatufre-400 F. her of important advantages. It permits theeconomic l o a 00 ammoniat =1.9:l. n Reatiou time 1 and highly eflinentextracuon and recoveryof copper values from mineral sulphides by aneasily conducted P sequence of operations. High capital and operatingcosts ercent Cu.

Partial Pressure of H2, p. s. i. precipitated and metal losses usuallyattendant wlth conventional V ;gfgg- 7o pyrometallurgical process areavoided. The method has the further important advantage that it can beemployed to treat mineral ores.and concentrates which contain values ofother metals and the method can be modified to provide'for the recoveryof such other metal values 15' separately from the copper values.

What I claim as new and desire to protect by Letters Patent of theUnited States is:

1. In a method of producing copper metal powder from copper bearingmineral sulphides in which finely divided copper bearing mineralsulphides are leached with a strong aqueous ammonia leach solution undera partial pressure of oxygen to produce a leach solution which containsdissolved copper values, free ammonia, and at least one oxidizablesulphur compound of the group consisting of polythionates andthiosulphate, undissolved residue is separated from the leach solutionand the clarified solution is reacted with a sulphur-free reducing gasto precipitate copper metal powder, the improvement which comprises thestep of reacting the clarified copper bearing leach solution prior toreacting it with a sulphur-free reducing gas with an oxygen bearing,oxidizing gas for a period of time sufficient to convert substantiallyall the oxidizable sulphur compounds to an oxidized sulfur compound ofthe group consisting of ammonium sulphate and ammonium sulphamate.

2. The method according to claim 1 characterized in that free ammonia ispresent in the clarified leach solution from the leaching step in excessof about 2 mols of ammonia per mol of dissolved copper and the ammoniato copper ratio is adjusted to about 2 mols of ammonia per mol of copperafter the oxidation of oxidizable sulphur compounds and prior to thestep of reacting the solution with a sulphur-free reducing gas.

3. The method according to claim 1 characterized in that the clarifiedleach solution subjected to reaction with an oxygen bearing, oxidizinggas for the oxidation of oxidizable sulphur compounds contains fromabout 200 to about 400 grams of ammonium sulphate per litre.

References Cited in the file of this patent UNITED STATES PATENTSHirs-ching Aug. 21, 1900 Hunt Apr. 29, 1902 Collins June 10, 1902 LeSueur Mar. 22, 1904 Cox May 15, 1917 Terry Mar. 23, 1920 Browning Feb.21, 1922 Laist July 17, 1923 Berthelot Sept. 25, 1923 Muller Oct. 2,1928 Marx et al. Dec. 2, 1930 Serciron Oct. 3, 1939 Keller Dec. 11, 1945Forward Nov. 27, 1951 Allen et :al. Aug. 4, 1953 Kenny et al Aug. 31,1954 Mackiw Nov. 2, 1954 Mackiw et al Nov. 2, 1954 FOREIGN PATENTS GreatBritain of 1854 Great Britain Aug. 2, 1928 OTHER REFERENCES I. of Chem.Education, vol. 6, No. 12, December 1929,

1. IN A METHOD OF PRODUCING COPPER METAL POWDER FROM COPPER BEARINGMINERAL SULPHIDES IN WHICH FINELY DIVIDED COPPER BEARING MINERALSULPHIDES ARE LEACHED WITH A STRONG AQUEOUS AMMONIA LEACH SOLUTION UNDERA PARTIAL PRESSURE OF OXYGEN TO PRODUCE A LEACH SOLUTION WHICH CONTAINSDISSOLVED COPPER VALUES, FREE AMMONIA, AND AT LEAST ONE OXIDIZABLESULPHUR COMPOUND OF THE GROUP CONSISTING OF POLYTHIONATES ANDTHIOSULPHATE, UNDISSOLVED RESIDUE IS SEPARATED FROM THE LEACH SOLUTIONAND THE CLARIFIED SOLUTION IS REACTED WITH A SULPHUR-FREE REDUCING GASTO PRECIPITATE COPPER METAL POWDER, THE IMPROVEMENT WHICH COMPRISES THESTEP OF REACTING THE CLARIFIED COPPER BEARING LEACH SOLUTION PRIOR TOREACTING IT WITH A SULPHUR-FREE REDUCING GAS WITH AN OXYGEN BEARING,OXIDIZING GAS FOR A PERIOD OF TIME SUFFICIENT TO CONVERT SUBSTANTIALLYALL THE OXIDIZABLE SULPHUR COMPOUNDS TO AN OXIDIZED SULFUR COMPOUND OFTHE GROUP CONSISTING OF AMMONIUM SULPHATE AND AMMONIUM SULPHAMATE.